Light-emitting diodes (LEDs) are attractive replacement candidates for conventional light sources based on incandescent bulbs and fluorescent light tubes. LEDs have higher energy conversion efficiency than incandescent lights and substantially longer lifetimes than both incandescent and fluorescent light fixtures. In addition, LED-based light fixtures do not require the high voltages associated with fluorescent lights. Finally, white LEDs with light conversion efficiencies significantly greater than those of fluorescent light tubes have been demonstrated in the laboratory and are now becoming commercially available.
Unfortunately, LEDs having outputs equivalent to that of a large conventional light source are not commercially available; hence, high power LED sources require that a large number of individual LEDs be combined to provide the desired output. The LEDs can be individually packaged and mounted on a separate substrate such as a printed circuit board or the like. Alternatively, a package in which a number of dies are mounted together and then encapsulated can be utilized.
The first solution leads to a light source that is significantly larger and more expensive than the second solution. The individually packaged LEDs typically consist of a die in a reflective cup that is encapsulated in a clear layer of material that protects the die from environmental attack. The cup collects light that leaves the sides of the die and redirects that light to the forward direction. In addition, each package must provide some mechanism that moves the heat generated by the LED to the outer surface of the package that is in contact with the printed circuit board on which the package die is eventually mounted. Hence, the packaged LED is significantly larger than the die. In addition, the package represents a significant fraction of the final cost of the packaged die. In addition, the manufacturer of the final product in which the light source is incorporated is forced to assemble the multiple packaged parts and provide a design based on multiple dies instead of the single conventional light source that the LED source is replacing.
Hence, a solution having a single final packaged part is often preferred. Unfortunately, the multi-die packages currently available have a number of problems. Inexpensive packages based on encapsulated lead frames typically provide space for only a few dies at most. The dies are mounted on one or more leads within the package. The heat generated by the dies is moved through the lead on which the dies are mounted, and hence, the heat removal capacity is limited by the heat moving capacity of one of the leads. Since the cross-sectional area of a lead is relatively small, the heat removal capacity is also limited. Various schemes for reducing the thermal resistance of the mounting lead have been utilized or proposed; however, the package's heat removal capacity is limited by the number of dies that can be mounted within a single small lead frame package.
Arrangements in which a large number of dies are packaged together in an extended light source having a specific configuration are also known. For example, U.S. patent application Ser. No. 11/618,459 (now published as U.S. Patent Application Publication No. 2008015886A1) discloses a linear light source in which a large number of LED dies are mounted on a substrate within a large package to provide a high power light source. Unfortunately, the cost of such light sources is prohibitive for many applications. In addition, the shape of the light source is fixed. If, for example, a light source in which the dies are arranged in a ring configuration were required, the light source would need to be almost totally redesigned.